u n i ve r s i t y o f co pe n h ag e n

Københavns Universitet

Zirconia- versus metal-based, implant-supported abutments and crowns

Hosseini, Mandana

Publication date:2012

Document versionPeer reviewed version

Citation for published version (APA):Hosseini, M. (2012). Zirconia- versus metal-based, implant-supported abutments and crowns: Comparativestudies on fracture mode and short-time clinical outcome. (1 ed.) København: Grafisk - København universitet.

Download date: 14. Mar. 2020

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S E C T I O N O F O R A L R E H A B I L I T A T I O N

D E P A R T M E N T O F O D O N T O L O G Y

F A C U L T Y O F H E A L T H A N D M E D I C A L S C I E N C E S

U N I V E R S I T Y O F C O P E N H A G E N

I N S T I T U T E O F D E N T A L M A T E R I A L S

O S L O

Zirconia- versus metal-based, implant-supported

abutments and crowns

Comparative studies on fracture mode and short-time clinical outcome

Mandana Hosseini

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CONTENTS

PREFACE ................................................................................................................................................................. 4

ACKNOWLEDGEMENT......................................................................................................................................... 5

ENGLISH SUMMARY ............................................................................................................................................. 6

DANSK RESUMÉ ..................................................................................................................................................... 8

ABBREVIATIONS ..................................................................................................................................................10

INTRODUCTION ....................................................................................................................................................11

ABUTMENT AND CROWN MATERIALS IN IMPLANT DENTISTRY ...................................................................................11

CERAMICS ..............................................................................................................................................................12

YTTRIA-STABILIZED TETRAGONAL ZIRCONIA POLYCRYSTALS (Y-TZP) .....................................................................13

BIOMECHANICAL AND TECHNICAL ASPECTS .............................................................................................................13

BIOLOGICAL ASPECTS .............................................................................................................................................14

AESTHETIC ASPECTS ...............................................................................................................................................15

PATIENT-RELATED ASPECTS ....................................................................................................................................20

AIMS ........................................................................................................................................................................21

MATERIAL AND METHODS ................................................................................................................................22

FRACTURE MODE DURING CYCLIC LOADING (STUDY I) ..............................................................................................23

DESIGN OF CLINICAL STUDIES (STUDY II, III AND IV) ...............................................................................................25

FOLLOW-UP EXAMINATIONS (STUDY III AND IV) ......................................................................................................25

BIOLOGICAL VARIABLES (STUDY III AND IV) ...........................................................................................................28

REPRODUCIBILITY OF RADIOGRAPHIC MEASUREMENTS (STUDY III)...........................................................................28

BIOMECHANICAL AND TECHNICAL VARIABLES (STUDY III AND IV) ...........................................................................28

AESTHETIC VARIABLES, PROFESSIONAL-REPORTED (STUDY II, III AND IV) ................................................................29

VALIDITY AND RELIABILITY OF COPENHAGEN INDEX SCORE (STUDY II)....................................................................30

PATIENT-REPORTED OUTCOMES (STUDY II, III AND IV) ............................................................................................30

STATISTICAL ANALYSIS...........................................................................................................................................31

Study I ...............................................................................................................................................................31

Study II ..............................................................................................................................................................31

Reliability ...................................................................................................................................................................... 31

Validity .......................................................................................................................................................................... 31

Study III and IV .................................................................................................................................................31

Patient-reported outcome ...................................................................................................................................32

SUMMARY OF RESULTS .....................................................................................................................................33

STUDY I .................................................................................................................................................................33

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STUDY II ................................................................................................................................................................35

Reliability ..........................................................................................................................................................35

Validity ..............................................................................................................................................................35

Patient- and professional-reported aesthetic outcomes .......................................................................................35

STUDY III ...............................................................................................................................................................38

Biological variables ...........................................................................................................................................38

Biomechanical and technical variables...............................................................................................................39

Professional-reported aesthetic variables ...........................................................................................................39

Patient-reported variables .................................................................................................................................39

STUDY IV ...............................................................................................................................................................40

Biological variable ............................................................................................................................................40

Biomechanical and technical variables...............................................................................................................40

Professional-reported aesthetic variables ...........................................................................................................41

Patient-reported aesthetic variables ...................................................................................................................41

DISCUSSION ...........................................................................................................................................................42

INFLUENCE OF RESTORATION MATERIALS ON PERI-IMPLANT TISSUE ..........................................................................42

Implant survival and success rate .......................................................................................................................42

Marginal bone loss and plaque accumulation .....................................................................................................42

Biological complications ....................................................................................................................................43

BIOMECHANICAL AND TECHNICAL COMPLICATION AT DIFFERENT RESTORATION MATERIALS .....................................44

Crown and abutment survival and failure rates ..................................................................................................44

Loss of retention, cement excess .........................................................................................................................45

Marginal adaptation ..........................................................................................................................................46

Veneering fracture .............................................................................................................................................46

Laboratory test method ......................................................................................................................................46

Fracture mode ...................................................................................................................................................47

AESTHETIC PARAMETERS AT DIFFERENT CROWN AND ABUTMENT MATERIALS ...........................................................48

Reliability of aesthetic parameters .....................................................................................................................48

Validity of aesthetic parameters .........................................................................................................................49

Professional-reported aesthetic outcome ............................................................................................................50

PATIENT-REPORTED OUTCOME ................................................................................................................................51

CONCLUSION ........................................................................................................................................................51

REFERENCE LIST .................................................................................................................................................53

STUDY I

STUDY II

STUDY III

STUDY IV

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PREFACE

The present thesis is based on the following studies, which will be referred to in the text by their

Roman numerals (I-IV):

Study I Hosseini M, Kleven E, Gotfredsen K. Fracture mode during cyclic

loading of implant-supported single-tooth restorations. Submitted.

Study II Hosseini M, Gotfredsen K. A feasible, aesthetic quality evaluation of

implant-supported single crowns: an analysis of validity and reliability.

Clin.Oral Implants.Res. MAR 2011. Epub ahead of print.

Study III Hosseini M, Worsaae N, Schiødt M, Gotfredsen K. A comparative, three-

year prospective study of implant-supported, single-tooth restorations of

all-ceramic and metal-ceramic materials in patients with tooth agenesis.

Submitted.

Study IV Hosseini M, Worsaae N, Schiødt M, Gotfredsen K. A one-year

randomised, controlled trial of implant-supported, single-tooth

restorations based on zirconia versus metal-ceramic. Submitted.

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ACKNOWLEDGEMENT

I would like to express my sincere gratitude to my main supervisor Professor Klaus Gotfredsen for

introducing me to scientific research, sharing his invaluable knowledge and for his generous support

to accomplish this work. Your patience and guidance through this project is deeply appreciated.

I wish also to thank co-supervisor chief dentist Nils Worsaae for sharing his extensive

knowledge of the field of oral surgery and for taking time to give scientific and practical advices to

my studies. More importantly, thank you for your kind friendship. My appreciations are also given

to co-supervisor chief dentist Morten Schiødt for scientific advices and for always showing interest

in my work. Alireza Sahafi is kindly appreciated for introducing me to the laboratory test method

and the equipments. I am also grateful to Erik Kleven for his fantastic encouragement and technical

supports during my research period in Oslo. My gratitude must be extended to Professor Jon E.

Dahl for reviewing the study I article and to all other staff at the Nordic Institute of Dental Materials

(NIOM) for their scientific and practical helps in Oslo.

All colleagues at the Special Clinic for Oral Rehabilitation are gratefully acknowledged for their

encouraging support and practical assistance with the clinical studies. Laboratory technicians Vivi

Rønne and Liselotte Larsen are appreciated for valuable practical help with the laboratory study in

Copenhagen. My great thanks go also to all other staff at Department of Oral Rehabilitation,

especially to chief dentists Betty Holm and Lone Pedersen Forsberg for their supports, kindness and

friendship. Dentists Stephen Ambrosius Pedersen and Anne Dorthe Frederiksen are greatly

appreciated for their kind affords and cooperation during clinical studies.

All participants in the clinical studies and all dental technicians in dental laboratories Flügge

Dental and DPNOVA are deeply appreciated. Ulrik Nikolaj Møller Hansen is also appreciated for

help with data collections in the clinical studies.

Associate Professor Lene Theil Skovgaard at Department of Biostatistics is kindly appreciated

for all the valuable statistical advices in a pedagogical way.

My great thanks must be given to my dear friend Azam Bakhshandeh for her trustful friendship

and advices, and for sharing her precious experiences during these three years of study. My dear

family is deeply appreciated for their support by excellent understanding and to give me all the time

and love I needed during this work.

Finally and above all, I would specially like to thank my beloved husband Ali for his continual

wonderful support and never-ending patience. Your kindhearted care to provide a peaceful

environment for me to work is priceless.

This project was financially supported by the Astra Tech®, Sweden; School of Dentistry in

Copenhagen; Faculty of Health Science, University of Copenhagen; The Danish Society for Oral

Implantology (DSOI), KOF/Calcin Foundation of The Danish Dental Association

(Tandlægeforeningen) and KaVo Everest GmbH, Germany.

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ENGLISH SUMMARY

To restore oral functions in patients with missing teeth, single-tooth implants are a well-documented

treatment option. Along with high survival rates, aesthetic factors have become an important

clinical outcome variable for evaluating treatment success of implant-supported restorations. Thus,

the selection of restoration materials should be based on proper optical characteristics in addition to

biocompatibility and sufficient strength of materials. Abutments and crowns based on zirconia are

one of the most recent alternatives to metal abutments and metal-ceramic crowns. To date, only few

comparative studies have reported on aesthetic, biological, biomechanical and patient-reported

outcomes of implant-supported single-tooth restorations of various biomaterials.

The aim of the present thesis was to investigate the clinical performance of zirconia-based

implant-supported single-tooth restorations and to estimate long-term biomechanical results of

zirconia-based versus metal-based restorations. The aim of study I was to analyse the mode of

fracture and number of cyclic loadings until veneering fracture of zirconia-based all-ceramic

restorations compared to metal-ceramic restorations. The aim of study II was to test the reliability

and validity of six aesthetic parameters used at the Copenhagen Dental School to assess the

aesthetic outcome of implant-supported restorations. The aims of study III and IV were to compare

the influence of different abutment and crown materials on biological, biomechanical and technical,

and professional- and patient-related aesthetic outcomes of implant-supported single-tooth

restorations.

In the first study, the most frequent fracture mode was the veneering fracture, which was more

severe at the all-ceramic than at the metal-ceramic restorations. Furthermore, more loading cycles

until veneering fracture were registered at the metal-ceramic than at the all-ceramic restorations.

In study II, the overall intra- and inter-observer agreements for the six aesthetic parameters were

substantial and moderate, respectively. The mucosal discolouration score had the highest intra- and

inter-observed agreement. The six aesthetic parameters had a highly significant correlation to the

corresponding VAS scores; thus, each parameter was found to be valid.

In study III and IV, all implants survived and the marginal bone loss was generally low. No

significant differences in the mPlI and mBI at restorations of different abutment materials and in the

marginal bone loss at restorations with zirconia and titanium abutments were recorded. In study III,

the marginal bone loss at restorations with gold alloy abutments was significantly higher than at

restorations with zirconia abutments. In study III and IV, the marginal adaptation of crowns was

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significantly less optimal at the all-ceramic than at the metal-ceramic crowns. The loss of retention

was the most frequent biomechanical complication and was mostly registered at the posterior

regions. The veneering fracture was slightly more frequent at the all-ceramic than at the metal-

ceramic crowns. The crown colour match was significantly better at all-ceramic versus metal-

ceramic crowns, while no significant difference in the other aesthetic parameters between various

restoration materials were observed. The patient-reported satisfaction with aesthetic outcomes was

not significantly different at restoration of various materials, and it was not significantly correlated

to the professional-reported aesthetic outcomes.

Conclusion: The biological outcome variables were similar at the different abutment materials;

however, the marginal bone loss was higher at the gold alloy compared to the zirconia and titanium

abutments. The biomechanical and technical outcome variables were more optimal at the metal-

ceramic than at the zirconia-based all-ceramic restorations. The six aesthetic parameters used in our

studies were feasible, reliable and valid, which make them useful for quality control of implant-

supported single-tooth restorations. The use of these aesthetic parameters indicated no remarkable

difference in aesthetic outcome of restorations with various abutments materials, but the all-ceramic

crowns provided a better colour match than the metal-ceramic crowns. The patients did not notice

difference in the aesthetic results of restorations of various materials.

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DANSK RESUMÉ

Behandling med enkelttandsimplantater er en veldokumenteret behandlingsform for at genskabe de

orale funktioner hos patienter med manglende tænder. De æstetiske faktorer er, ud over en høj

overlevelsesprocent, blevet vigtige klinisk variabler for vurdering af behandlingssuccesen af

implantatunderstøttede restaureringer. Derfor bør valget af restaureringsmaterialer være baseret på

deres optiske egenskaber foruden deres biokompatibilitet og tilstrækkelig materialestyrke.

Abutment og kroner baserede på zirkonia er et af de nyeste alternativer til metal abutment og

metalkeramiske kroner. I dag er der kun få studier, der har sammenlignet og rapporteret de

æstetiske, biologiske, biomekaniske og patientrelaterede resultater af implantatunderstøttede

enkelttandsrestaureringer af varierende biomaterialer.

Formålet med denne afhandling var at undersøge den kliniske ydeevne af zirkonia-baserede

implantatunderstøttede enkelttandsrestaureringer, og at estimere biomekaniske langtidsresultater af

zirkonia-baserede versus metalkeramiske restaureringer. Formålet med studie 1 var at sammenligne

frakturmønstret og antallet af cykliske belastninger indtil fraktur af påbrændingskeramikken mellem

zirkonia-baserede og metal-baserede restaureringer. Formålet med studie 2 var at undersøge

pålideligheden og gyldigheden af seks æstetiske parametre, som var anvendt til vurdering af det

æstetiske resultat af implantatunderstøttede restaureringer på Tandlægeskolen i København.

Formålene med studie 3 og 4 var at sammenligne effekten af forskellige abutment- og

kronematerialer på de biologiske, biomekaniske og tekniske samt på de professional- og patient-

rapporterede æstetiske resultater af implantatunderstøttede enkelttandsrestaureringer.

Ved den første studie var fraktur af påbrændingskeramikken den hyppigste frakturmønster, som

var mere omfattende ved de helkeramiske end ved de metalkeramiske restaureringer. Desuden var

flere belastningscykler indtil fraktur af påbrændingskeramikken registreret ved de metalkeramiske

end ved de helkeramiske restaureringer.

I studie 2 var den overordnede intra- og inter-observatør enighed for de seks æstetiske parametre

henholdsvis substantiel og moderat. Den højeste intra- og inter-observatør enighed var registreret

ved scoren for misfarvning af mukosa. De seks æstetiske parametre havde en signifikant korrelation

til de tilsvarende VAS scorer og var derfor gyldige.

I studie 3 og 4 overlevede alle implantater og marginalt knoglesvind var generelt lavt. Forskellen

i mPlI og mBI var ikke signifikant ved restaureringer af forskellige abutment materialer, og heller

ikke forskellen i marginalt knoglesvind var signifikant mellem restaureringer med zirkonia og titan

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abutment. I studie 3 var marginalt knoglesvind signifikant større ved restaureringer med

guldlegering end ved dem med zirkonia abutment. I studie 3 og 4 var marginalt tilpasning af

zirkonia-baserede helkeramiske kroner mindre optimal end de metalkeramiske kroner. Den

hyppigste biomekaniske komplikation var kroneløsning, som ofte var registreret i de posterior

regioner. Frakturen af påbrændingskeramikken var lidt hyppigere ved de helkeramiske end ved de

metalkeramiske kroner. Kronefarven var signifikant bedre ved de helkeramiske end ved de

metalkeramiske kroner, mens de andre æstetiske parametre ikke var signifikant forskellige ved

diverse restaureringsmaterialer. Patienternes tilfredshed med det æstetiske resultat var ikke

signifikant forskellig ved restaureringer af forskellige materialer og var ikke signifikant korreleret

til de professionelles vurdering af det æstetiske resultat.

Konklusion: De biologiske resultater var sammenlignelige ved forskellige abutment materialer,

dog var marginalt knoglesvind større ved abutment af guldlegering end ved zirkonia abutment. De

biomekaniske og tekniske resultater var mere gunstige ved de metalkeramiske end ved de zirkonia-

baserede helkeramiske restaureringer. De seks æstetiske parametre brugt i vores studier var

gennemførlige, pålidelige og gyldige, hvilket gør dem anvendelige for kvalitetskontrol af

implantatunderstøttede enkelttandsrestaureringer. Anvendelse af disse parametre viste ingen

betydelige forskelle i det æstetiske resultat af restaureringer med forskellige abutment materialer,

men de helkeramiske kroner udviste en bedre farvematch end de metalkeramiske kroner.

Patienterne bemærkede ikke forskel i det æstetiske resultat mellem restaureringer af forskellige

materialer.

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ABBREVIATIONS

AC All-ceramic

AC-C All-ceramic crown on ceramic abutment

AC-M All-ceramic crown on metal abutment

CAD/CAM Computer-aided design/ Computer-aided manufacturing

CDA California Dental Association

CEI Complex Esthetic Index

CIS Copenhagen Index Score

DES Mesio-distal distance in edentulous space

ICA Implant Crown Aesthetic index

ISP Implant-supported premolar crown

ISSC Implant-supported single crown

mBI Modified sulcus Bleeding Index

MC Metal-ceramic

MC-M Metal-ceramic crown on metal abutment

mPlI Modified Plaque Index

OHIP Oral Health Impact Profile

PES Pink Esthetic Score

PPD Probing Pocket Depth

RCT Randomised Clinical Trial

VAS Visual Analogue Scale

WES White Esthetic Score

Y-TZP Yttria-stabilized tetragonal zirconia polycrystal

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INTRODUCTION

In modern prosthetic dentistry, the major purpose is to assure oral function for the individual

patient. Oral functions include mastication, aesthetics and psycho-social abilities, occlusal support

and dental arc stability, and other functions such as tactile perception, phonetics and taste 1.

To restore oral functions in patients with missing teeth, single-tooth implants are a well-

documented treatment option 2-4

. Along with good survival rates, aesthetic factors have become an

important clinical outcome variable for evaluating treatment success of implant-supported

restorations 5. When restoring missing teeth with implant-supported restorations in aesthetic

demanding regions, selection of abutment and crown materials is one of the possibilities to achieve

an optimal aesthetic result. Abutments and crowns based on zirconia are one of the most recent

alternatives to metal abutments and metal-ceramic crowns. It may be hypothesised that implant-

supported single-tooth restorations with zirconia abutments and all-ceramic crowns will result in a

better clinical outcome than metal abutments and metal-ceramic crowns, but to confirm it

comparative studies have to be performed. To date, only few comparative studies have reported on

aesthetic, biological, biomechanical and patient-reported outcomes of implant-supported, single-

tooth restorations of various biomaterials.

Abutment and crown materials in implant dentistry

Titanium, gold alloys and oxide ceramics are the abutment material options in implant dentistry 6.

Traditionally, implant-supported restorations included titanium abutments and metal-ceramic

crowns. Metal abutments have been suggested to shine through mucosa and induce a greyish

appearance of peri-implant soft tissue 6-14

. To improve the aesthetic outcome and to enhance the

colour harmony between restorations and natural dentition, high-strength oxide ceramics, mainly

zirconia, has been introduced as implant abutment and crown core materials 5, 7, 8, 10, 13, 15, 16

. Due to

opaque colour of oxide ceramics, crown core of zirconia must be veneered with more translucent

ceramic materials to imitate the natural tooth colour. However, only few in vitro and in vivo studies

have compared implant-supported, single-tooth restorations of metal-ceramic materials with

zirconia-based abutments and crowns 17, 18

.

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Ceramics

The increased requirement of improving aesthetic properties of restorations is one of the principal

driving forces behind a rapid development of tooth-coloured dental restorative materials 19, 20

.

Ceramic materials represent one of the few choices for tooth-coloured restorative treatments and are

considered as one of the most biocompatible dental materials with relative low incidence of

biological side effects 19, 21

.

Dental ceramics can be classified at a microstructure level by their composition of glass-to-

crystalline ratio (Table 1) 22

.

Table 1: Classification of dental ceramics

Classifications Main composition

Flexural

strength

(MPa)*

Glass-based systems Feldspathic porcelains Silica (SiO2) 70–100

Glass-based systems with

fillers

Low-to-moderate leucite-

containing feldspathic glass

Silica with fillers, fillers usually

crystalline (leucite, lithium-

disilicate, flourapatite)

120–300

High-leucite (~50%)-containing

glass, glass-ceramics

Lithium-disilicate (~70%) glass-

ceramics

Crystalline-based systems

with glass fillers Infiltration ceramics

Alumina or zirconia-toughened

alumina with glass fillers > 300

Polycrystalline solids Oxide ceramics Alumina (Al2O3) 275–700

Zirconia (ZrO2) 800–1500

* Milleding P, Karlsson S, Molin M. Dentala helkeramer i teori och klinik. Stockholm: Gothia, 2005.

Glasses in dental ceramics originate principally from a group of mined minerals called feldspar.

Feldspathic porcelains are primarily composed of silica (SiO2), alumina (Al2O3) and various amount

of K2O and Na2O 19, 22, 23

. In order to improve the mechanical properties of dental ceramics, crystals

have either been added to or grown in glass matrix (glass-ceramics). In general, more glass in the

microstructure results in more translucent ceramic, while more crystals gives more opaque

appearance. Glass-based ceramic materials are highly aesthetic materials with best optical

13

properties, and they tend to be employed as veneer materials for metal or ceramic substructures 22,

23.

Other ceramic materials are mainly used as substructures and have been developed to fulfil the

aesthetic and high mechanical requirements. Aluminium oxide (alumina) and zirconium dioxide

(zirconia) represents the high-strength, polycrystalline oxide ceramics with densely packed crystals

and no glassy components. Zirconia has a higher fatigue-crack propagation threshold than alumina,

and it has a fracture toughness that is at least twice higher than alumina 24

.

In clinical studies of implant-supported single-tooth restoration, fracture of veneering ceramics

has been reported as a frequent problem 2, 25, 26

. Only one recent in vitro study has investigated a

possible difference in veneering fracture of implant-supported single crowns (ISSCs) when glass-

ceramic and feldspathic porcelain were used 27

. Thus, more comparative in vitro and in vivo studies

of ISSCs with various types of veneering ceramics are needed.

Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP)

The superior mechanical properties of zirconia make this material suitable for biomedical

application, especially in implant dentistry 28

. The key factor for biomechanical properties of

zirconia is phase transformation of crystals 24

. The crystalline state of zirconia is monoclinic at

room temperature and occupying approximately 4.5% more volume than a tetragonal crystalline

state at firing temperature (1170oC to 2370

oC). The tetragonal form of crystals is stabilized at room

temperature by addition of Y2O3 (yttria) 28, 29

. The yttria-stabilized tetragonal zirconia polycrystals

(Y-TZP) is ―metastable‖, and stress-generating surface treatment such as grinding and sandblasting

as well as stress concentration at tip of a propagating crack are able to trigger transformation of

material back to the monoclinic state 29

. The subsequent increasing volume leads to surface

compression and increase flexural strength and susceptibility to aging 24, 29

. Low temperature

degradation of zirconia aggravated by presence of water is a well-documented phenomenon 23, 29, 30

.

However, only few long-term clinical studies of zirconia-based abutment and crowns supported by

implants are published 31

. Thus, the influence of the microstructural transformations of zirconia on

clinical performance of implant-supported single-tooth restorations is still unknown.

Biomechanical and technical aspects

During masticatory function, dental restorations are exposed to fatigue under repeated loading in

wet environment 32

. Repeated contact loading reduces the strength and limits the useful life-time of

14

all ceramic materials, which occurs mostly in veneering ceramics but even to some degree in Y-

TZP substructrures 33

. During cyclic loading test, brittle ceramic materials are in an active stress

intensity in a higher time-average than at static loadings under equivalent loading conditions;

therefore, cyclic loadings causes greater damage in ceramic materials 33

.

To study the biomechanical strength and the fracture mode of ceramic materials and in attempts

to link in vivo and in vitro studies, inclusion of intraoral conditions such as cyclic loadings and

presence of water to laboratory study protocols are required 32, 34

. Despite of these

recommendations, several in vitro studies of implant-supported restorations have used static load-

to-fracture tests 35-40

. Consequently, fractures of components such as screws and abutments have

frequently been reported 35-40

. As these laboratory study results are in contrast to clinical findings,

the clinical relevance of static load-to-fracture test protocol could be questioned 2, 25, 31, 34, 41, 42

.

In clinical studies of implant-supported restorations, veneering fracture has been reported as one

of the most common biomechanical complications 25, 26, 31, 43

. The clinical failures of all-ceramic

restorations are complex and involve both patient- and material-related variables 44

. To compare the

traditionally implant-supported restorations of metal abutments and metal-ceramic crowns with

restorations of zirconia abutments and zirconia-based all-ceramic crowns, the well-controlled

laboratory studies are useful to eliminate the inter-subject variability.

Biological aspects

Biocompatibility involves the effects of material on the medium and vice versa. Biomaterials and

their degradation products should not induce inflammatory reactions, allergic, immune, toxic,

mutagen or carcinogenic reactions 28

. Zirconia has been used as orthopaedic hip implant material in

more than 20 years 30, 45

, and the great biocompatibility of this material has been demonstrated in

various in vitro and in vivo studies 46

.

In implant dentistry, zirconia has mainly been used as an alternative material for metal abutments

and substructures of fixed prostheses. As the transmucosal part of abutment is located close to the

alveolar bone, the soft tissue integration and the marginal tissue reaction to the abutment material

is important for stability of the peri-implant bone level 47

. The abutment material has been shown to

influence the quality of epithelial attachment, and it has been indicated that the choice of abutment

material must be based on its ability to promote soft-tissue integration and maintain a healthy peri-

implant mucosa 48, 49

.

15

The well-known high biocompatibility of titanium abutments has been the major reason to use

this material as a ―golden standard‖ to compare biological properties of different abutment

materials. In an experimental study in dogs, zirconia abutments established a similar mucosal

attachment as titanium abutments, while gold alloy abutments achieved no proper soft tissue

integration 48

. Degidi et al. 50

compared inflammatory reactions in biopsies from peri-implant

tissues around zirconia and titanium healing caps, and they demonstrated more inflammatory

infiltrates in soft tissue around titanium than around zirconia healing caps. In the study by Welander

et al. 48

, the proportion of leucocytes at barrier epithelium was lower at zirconia abutments

compared to titanium abutments, and this difference was proposed to be related to variations in

bacterial plaque accumulation on titanium and zirconia abutment surfaces 48

. It is noteworthy that

oral plaque accumulation has been suggested to be one of the major reasons of implant failure 51

. In

in vivo studies by Rimondini et al. 45

and Scarano et al. 52

, significantly less accumulation of plaque

on zirconia compared to titanium surfaces was detected. Based on these observations, it was

suggested that zirconia was a suitable material for abutment fabrication 45, 52

.

However, clinical studies are valuable to compare the biological outcome of different abutment

materials. In a four-year prospective clinical study by Vigolo et al. 53

, no biological variation

between gold alloy and titanium abutments were detected, which is in contrast to the results of the

animal studies by Abrahamsen et al. 49

and Welander et al. 48

. In a 3-year randomized clinical study

by Zembic et al. 17

, the comparison of customized zirconia and titanium abutments demonstrated no

significant differences in biological parameters between zirconia and titanium abutments. Thus,

more comparative, clinical data are needed to draw a definite conclusion on effect of abutment

materials on peri-implant tissue.

Aesthetic aspects

Aesthetics seems to be one of the main reasons why patients seek prosthetic treatment 1. The

standards for aesthetic fixed implant prosthesis have been defined as healthy peri-implant tissues

with natural appearance of restorations in harmony with the healthy surrounding dentition 54, 55

.

Thus, restorations should be selected not only base on appropriate biological properties and

sufficient strength to withstand the masticatory forces but also based on proper optical

characteristics to provide an optimal aesthetic result 20, 56

.

The great long-term survival rate of single-tooth implants 3, 25, 31, 57, 58

is one of the reasons to

more focus on aesthetic outcome of implant-supported single crowns 59-61

. Since the development of

16

metal-ceramic crowns in the early sixties, these restorations have represented the ―golden standard‖

in prosthetic dentistry 62, 63

. A progressive introduction of high-strength oxide ceramics, especially

zirconia, with white colour and ―relative translucency‖, has generally resulted in an increased use of

metal-free restorations 64

. In implant dentistry, the use of zirconia-based, implant-supported

restorations is suggested to enhance the colour match of restorations with natural teeth and to

decrease the grayish appearance of peri-implant mucosa 20

. A high standard of aesthetic quality is

particularly important at implant-supported, single crowns as an immediate visual comparison of

the implant-supported crown with the surrounding natural dentition is possible 5, 65

. To have an

insight in the aesthetic result of a specific treatment and to facilitate analysis of results in order to

improve the prosthetic treatment, the use of rating scores with a division in different items have

been recommended 66

. Thus, a feasible, valid and reliable rating score is required to compare the

aesthetic outcome of zirconia-based, all-ceramic and metal-ceramic implant-supported, single-tooth

restorations.

In the dental literature, the California Dental Association (CDA) index 67

has frequently been

used 43, 68, 69

. This index includes five parameters, whereas two, i.e. anatomic form and colour match

are suitable to describe implant-supported single crowns. To describe the aesthetics of peri-implant

soft tissue, Jemt 70

introduced simple scores including the papilla index scores as well as scores for

presence or absence of soft tissue discolouration and presence or absence of visible titanium

margins.

To assess the aesthetic outcome of implant-supported single crowns, a number of other

categorical rating scores have been developed during the last decade (Table 2). Some of these rating

scores, e.g. Implant Esthetic Score 71

and Pink Esthetic Score 59

concentrate only on aesthetic

outcome of peri-implant tissue. Other scores, e.g. Implant Crown Aesthetic Index 66

and a score

comprised of modified Pink Esthetic Score and White Esthetic Score 72

as well as the scores used at

the Dental School in Copenhagen 73

include also the aesthetic parameters of implant-supported

restorations. Some of these scores, e.g. the Implant Crown Aesthetic Index, are very detailed and

comprehensive indices, but they appears to be the most difficult to use 74

. Additionally, reliability

and, in particular, validity of some of these rating scores have not been tested in clinical settings.

The test of reliability of scales is necessary in establishing the usefulness of a measure, but it is not

sufficient. The validity of scales should be determined to draw an accurate conclusion about the

presence and degree of the attribute. This could be performed by analysing the correlation of a scale

with a ‗golden standard‘, which has been used and accepted in the field 75

. Visual Analogue Scale

17

(VAS) is a continuous scale and has also been used to assess the aesthetic outcome of implants by

dentists in some studies 65, 76, 77

. As this scale has most often been used as a measuring instrument

for dental, dentofacial, or facial aesthetics 78

VAS could be used as a ―golden standard‖ to validate

the categorical aesthetic parameters.

18

Table 2. Overview of studies introducing categorical rating scores for aesthetic assessment of implant-supported single crowns

Study Index Parameters (number of scores) Reliability &

validity

Meijer et al. 66 Implant Crown Aesthetic Index (ICA)

- Mesiodistal dimension of the crown (5 scores)

- Position of the incisal edge of the crown (5 scores)

- Labial convexity of the crown (5 scores)

- Colour and translucency of the crown (3 scores)

- Surface of the crown (3 scores)

- Position of the labial margin of the peri-implant mucosa (3 scores)

- Position of mucosa in the approximal embrasures (3 scores)

- Contour of the labial surface of the mucosa (5 scores)

- Colour and surface of the labial mucosa (3 scores)

+ reliability 66, 79

- validity

Fürhauser et al. 59 Pink Esthetic Score (PES)

- Mesial papilla (3 scores)

- Distal papilla (3 scores)

- Level of soft tissue margin (3 scores)

- Soft tissue contour (3 scores)

- Alveolar process deficiency (3 scores)

- Soft tissue color (3 scores)

- Soft tissue texture (3 scores)

+ reliability 80

- validity

Testori et al. 71 Implant Aesthetic Score (IES)

- Presence and stability of the mesiodistal papilla (3 scores)

- Ridge stability bucco-palatally (2 scores)

- Texture of the peri-implant soft tissue (3 scores)

- Color of the peri-implant soft tissue (3 scores)

- Gingival contour (3 scores)

- reliability

- validity

Dueled et al. 73

Aesthetic of crowns

- Crown morphology (4 scores)

- Crown colour match (4 scores)

Facial aesthetic

- Symmetry/harmony (4 scores)

Aesthetic of mucosa

- Mucosal discolouration (4 scores)

- Mesial papilla (4 scores)

- Distal papilla (4 scores)

+ reliability 81

+ validity 81

19

Belser et al. 72 Modified PES and White Esthetic Score

(PES/WES)

Modified PES

- Mesial papilla (3 scores)

- Distal papilla (3 scores)

- Level of facial mucosa (3 scores)

- Curvature of facial mucosa (3 scores)

- Root convexity/ soft tissue color and texure (3 scores)

WES

- Tooth form (3 scores)

- Outline and volume of crown (3 scores)

- Color (hue and value) (3 scores)

- Surface texture (3 scores)

- Translucency and characterization (3 scores)

- reliability

- validity

Juodzbalys & Wang 82 Complex Aesthetic Index (CEI)

S

- Soft tissue contour variations (3 scores)

- Soft tissue vertical deficiency (3 scores)

- Soft tissue color and texture variations (3 scores)

- Mesial papillae appearance (3 scores)

- Distal papillae appearance (3 scores)

P

- Mesial interproximal bone (3 scores)

- Distal interproximal bone height (3 scores)

- Gingival tissue biotype (3 scores)

- Implant apico-coronal position (3 scores)

- Horizontal contour deficiency (3 scores)

R

- Color and translucency (3 scores)

- Labial convexity in the abutment/implant junction (3 scores)

- Implant/crown incisal edge position (3 scores)

- Crown width/length ratio (3 scores)

- Surface roughness and ridges (3 scores)

+ reliability 82

- validity

20

Patient-related aspects

The aesthetic outcome is a priori very subjective and should be assessed both by the patient and the

dentist. It has been recommended that dentist and patient should plan the aesthetic treatment

together 83, 84

. However, the aesthetic evaluations by the patient should be more focused than the

professional assessment 65, 74, 85

.

In a number of clinical studies of implant-supported restorations, questionnaires have been used

to register the patient-reported aesthetic outcome 3, 65, 69, 73, 74, 76, 86-88

. In these studies, the patients

judged the appearance of the restorations on either a VAS 3, 65, 69, 76, 87, 88

or on a categorical scale

varying from two to six scores 73, 74, 86, 87

. In spite of variations in aesthetic questions and their

assessment methods, it was generally indicated that the patients were highly satisfied with the

aesthetic result of their implant-supported single crowns 3, 69, 73, 74, 76, 86-88

, and the dental

professionals were more critical than patients on this outcome variable 73, 74, 76, 86-88

. Based on these

deviations between patient- and professional-reported aesthetic outcomes, it was proposed that the

subjective patient evaluation is of primary importance for the assessment of a successful outcome in

implant dentistry 89

. As one of the major reasons for using all-ceramic restorations is to improve the

aesthetic result, the assessment of patient´s opinion on appearance is even more important when

comparing implant-supported all-ceramic and metal-ceramic restorations. To date, only one

randomized clinical study has compared patient-reported aesthetic outcome of implant-supported,

all-ceramic with metal-ceramic single crowns 18

. Hence, more clinical studies are needed to

evaluate any differences in patient´s aesthetic satisfaction between implant-supported single crowns

of different materials.

21

AIMS

The specific objectives of the studies in the thesis were:

to compare the mode of fracture and number of cyclic loadings until veneering fracture

of all-ceramic and metal-ceramic restorations supported by implants (study I).

to test the reliability and validity of the aesthetic parameters used at the Copenhagen

Dental School and to compare the professional- and patient-reported aesthetic outcomes

(study II).

to compare the influence of abutments of zirconia (study III & IV), titanium (study III

& IV) and gold alloy (study III) on biological outcome variables of implant-supported

single-tooth restorations.

to compare the impact of all-ceramic and metal-ceramic restorations on biomechanical

and technical outcome variables of implant-supported, single-tooth restorations (study

III & IV).

to compare the impact of restoration materials on the professional- and patient-reported

aesthetical outcome variables of implant-supported, single-tooth restorations (study III

& IV).

22

MATERIAL AND METHODS

A summary of the material and methods used in the four studies is presented in Table 3.

Table 3. Design, number of subjects and materials used in the in vitro and in vivo studies

Study I Study III Study IV (&II*)

Study design In vitro

4.2 mill cyclic loadings

In vivo

3-year, prospective study

In vivo

1-year, randomized study

No. patients - 59 36

Mean age (range) - 27.9 (18-50) 28.1 (19-57)

No. of ISSCs 32 98 75

Implant position

- Incisors

- Canines

- Premolars

-Molars

-

-

-

-

42

26

29

1

-

-

75

-

Type of ISSCs

(no.)

AC

(16)

MC

(16)

AC

(52)

MC

(46)

AC

(38)

MC

(37)

Abutment

materials

(no.)

Zirconia

(16)

Titanium

(16)

Zirconia

(52)

Titanium

(21)

Gold alloy

(25)

Zirconia

(38)

Titanium

(35)

Gold alloy

(2)

Coping materials

(no.)

Zirconia

(16)

Gold alloy

(16)

Zirconia

(49)

Glass-ceramic

(3)

Gold alloy

(34)

Zirconia

(12)

Zirconia

(38)

Gold alloy

(37)

Veneering

ceramics

(no.)

Feldspathic

(8)

Glass-

ceramic

(8)

Feldspathic

(8)

Glass-

ceramic

(8)

Glass-ceramic

(52)

Glass-ceramic

(46)

Feldspathic

(34)

Glass-

ceramic

(4)

Feldspathic

(34)

Glass-

ceramic

(3)

* In study II, clinical photographs of 34 patients also participating in study IV were included.

ISSC: Implant-supported single crown

AC: All-ceramic ISSC

MC: Metal-ceramic ISSC

23

Fracture mode during cyclic loading (study I) In this in vitro study, 32 implant-supported single crowns (ISSCs) were inserted in acrylic resin

blocks. Two test groups of all-ceramic (AC) restorations and two control groups of metal-ceramic

(MC) restorations were prepared.

All abutments had prefabricated preparations and were prepared with an angle of 45 degrees at

the palatal aspect in the upper part. The abutments were tightened to the implants with a torque of

25 Ncm by using a torque wrench. All crowns were manufactured as canines with a palatal

inclination of 45 degree (Figure 1).

Based on a great number of pilot tests, fracture mode 0 (no fractures) to 7 (implant fracture) were

categorized (Table 4, Figure 2) and the test method was developed.

The study samples were subjected to cyclic loading in a test machine constructed to and used in

studies by Isidor et al. 90, 91

and Sahafi et al. 92

. The cyclic loadings were performed with a stainless

steel ball with a diameter of 6 mm directed to the palatal surface of the crowns, 1.5 mm below the

incisal edge. The loading angle was 15 degree to the long axis of implants. During the cyclic

loadings, the restorations were kept under humid conditions with distilled water. The loading force

was set to 800 N with a frequency of 2 Hz. and continued to 4.2 million cycles or until fracture of

copings, abutments or implants. The number of cyclic loadings and fracture modes were recorded.

The first recorded fracture mode was the initial fracture, and the final fracture mode was the

fracture after 4.2 mill cycles or the fracture of coping, abutment or implant.

Figure 1. Illustration of an implant insertion in

acrylic block (1a), prepared abutment (1b) and

the crown design (1c and 1d)

24

Table 4. Fracture modes

Fracture modes Descriptions by visual examination

0 No fractures, flaws or infractions

1 Infraction in veneering ceramic

2 Chip-off within veneering ceramic (Cohesive fracture)

3 Fracture of veneering ceramic with exposure of coping

Fracture mode 3.a: fracture <½ of veneering ceramic

Fracture mode 3.b: fracture ≥ ½ of the veneering ceramic

4 Fracture of both veneering ceramic and coping without abutment fracture

5 Fracture of coping and abutment

6 Fracture of abutment without crown fracture

7 Fracture of implant

Figure 2. Illustration of the fracture modes. The numbers indicate the

corresponding fracture mode.

25

Design of clinical studies (study II, III and IV)

The clinical studies (study II, III & IV) included patients with tooth agenesis referred to the School

of Dentistry in Copenhagen for prosthetic treatments. The inclusions criteria were all patients, who

required replacements with implant-supported, single crowns (ISSCs), had no contraindications for

oral implant treatment, e.g., uncontrolled diabetes, metabolic bone disorders, history of radiotherapy

in head and neck, current chemotherapy or other diseases with an influence on bone healing, and

participated in 1-year (study II & IV) and 3-year (study III) follow-up examinations.

All implants (Astra Tech®, Mölndal, Sweden) were inserted at The Department of Oral and

Maxillofacial Surgery, Glostrup University Hospital (Copenhagen, Denmark). After an implant

healing period of 4–6 months, the prosthetic procedures were initiated at the School of Dentistry in

Copenhagen.

In study III, the patients were consecutively included between 2005 and 2008. The treating

prosthodontists decided the use of AC and MC restorations. Fifty-nine patients; 35 women and 24

men; fulfilled the inclusion criteria and were rehabilitated with 98 ISSCs; 52 AC and 46 MC

restorations.

In study IV, all patients had tooth agenesis in the premolar regions, and the prosthetic treatments

were randomised between AC and MC restorations. The study protocol was accepted by the Danish

Regional Committee on Biomedical Research Ethics. Thirty-six patients (18 men and 18 women)

were included and restored with 75 ISSCs; 38 AC and 37 MC restorations.

In study II, the clinical photographs of 66 ISSCs of 34 out of the 36 patients in the study IV were

included to assess the aesthetic outcomes.

Follow-up examinations (study III and IV)

The patients were recalled to baseline (study III & IV), 1-year (study IV) and 3-year (study III)

follow-up examinations. The clinical and radiological registrations were performed, and biological,

biomechanical and technical, aesthetic and patient-reported variables were recorded (Table 5).

26

Table 5. Outcome variables registered at the clinical studies (III & IV)

Variables Description Baseline Follow-up ≥1 year Study

Biological Implant survival1) Implants still in function X X III & IV

Implant mobility1) Clinical absence of mobility X X III & IV

- Modified Plaque Index (mPlI) 2)

- Modified Sulcus Bleeding Index

(mBI)2)

Median values of mPlI and mBI scores assessed at four sites

of each ISSC

X

X

X

X III & IV

Complications

- Neurosensory disturbance

- Devitalisation of adjacent teeth

- Inflammatory reactions; fistula, exudation/suppuration or pain

X

X

X

X

X

X

III & IV

- Marginal bone loss≥ 2 mm - X III

- Marginal bone loss≥ 1.6 mm

- PPD≥ 5mm

- X IV

X X

Marginal bone level Most coronal bone-implant contact mesially and distally X X III & IV

Marginal bone loss Mean value of change in mesial and distal marginal bone

level - X III & IV

Interproximal marginal bone width Mean values of distance between neighbouring teeth and

implants X III

Orthodontic pretreatment Orthodontic treatment before implant insertion X - III

Apical root resorption of adjacent

teeth3)

Absence: apical root score 0 and 1

Presence: score 2, 3 or 4 X III

27

Biomechanical

and technical

Crown and abutment survival Crowns and abutments still in function X X III & IV

Cement excess Radiopaque particles detected on radiographs at the ISSCs X X III & IV

Marginal adaptation4) Radiological evaluation of marginal fit of the crowns, score

1 to 4 X III & IV

Complications

- Loosening or fracture of the abutment screws

- Loss of retention

- Fracture including chipping of the veneering ceramics

X

X

X

X

X

X

III & IV

Aesthetic

Copenhagen Index Score

- - Crown morphology score

- - Crown colour match score

- - Mucosal discolouration score

- - Papilla index score, mesially

- - Papilla index score, distally

Each of five parameters: score 1 to 4 4)

X

X III & IV X

X

X

X

X

X

CIS (summary score) Overall professional-reported aesthetic outcome X X

Mesio-distal distance in the edentulous

space (DES)

Minimum coronal distance between the proximal surfaces

facing to the implant site X - III & IV

Patient-reported

Danish version of Oral Health Impact

Profile questionnaire5) (OHIP-49)

Aesthetic outcome: summary of scores from question 3, 4,

20, 22, 31 and 384)

Masticatory outcome: summary of scores from question 1,

28, 29 and 326)

Overall impact of oral health on life quality: summary score

of 49 questions

X

X

X

X

X

X

III

Visual Analogue Scale (VAS) Overall aesthetic outcome of each restoration - X IV

1) Albrektsson, T. & Isidor, F. Consensus report of session IV. In: Lang NP, Karring T, eds. Proceedings of the 1st European workshop on periodontology. Berlin: Quintessence Publ.Co.Ltd. 1993; 365-369

2) Mombelli A, van Oosten MA, Schurch E Jr, Land NP. The microbiota associated with successful or failing osseointegrated titanium implants. Oral Microbiol.Immunol. 1987; 2:145-151

3) Malmgren, O., Goldson, L., Hill, C., Orwin, A., Petrini, L. & Lundberg, M. (1982) Root resorption after orthodontic treatment of traumatized teeth. Am.J.Orthod. 82: 487-491.

4) Dueled E, Gotfredsen K, Trab DM, Hede B. Professional and patient-based evaluation of oral rehabilitation in patients with tooth agenesis. Clin.Oral Implants.Res. 2009; 20:729-736

5) Gjørup, H., Svensson, P. OHIP-(D), en dansk version af Oral Health Impact Profile, Tandlægebladet 2006; 4:304-311

6) Goshima, K., Lexner, M.O., Thomsen, C.E., Miura, H., Gotfredsen, K. & Bakke, M. Functional aspects of treatment with implant-supported single crowns: a quality control study in subjects with tooth agenesis. Clin.Oral

Implants.Res. 2010; 21: 108-114.

28

Biological variables (study III and IV)

The clinical examinations included registration of implant survival and mobility, the modified

Plaque Index (mPlI) and the modified Sulcus Bleeding Index (mBI) 93

at four aspects of each

ISSCs.

Furthermore, complications were registered (study III & IV) and it was recorded whether or not

the patients had received orthodontic treatments previously (study III).

The radiological assessments of marginal bone level (study III & IV) and interproximal marginal

bone widths (study III) were performed (Figure 3). The absence or presence of apical root

resorption of the neighbouring teeth was recorded 94

(study III).

Reproducibility of radiographic measurements (study III)

To estimate the intra-observer reproducibility, the radiographs of 20 included implants were

randomly selected, and the mesial and the distal marginal bone levels at the baseline and the 3-year

examination were re-examined four weeks after the first assessments. The mean difference between

80 repeated measurements was 0.02 mm, SD 0.34; thus, the "limits of agreement" varied from -0.65

to + 0.69 mm (i.e., 95% of the differences in the repeated measurements are expected to lie within

this interval).

Biomechanical and technical variables (study III and IV)

The clinical examinations included crown survival and registration of loosening or fracture of the

abutment screws, loss of retention and fracture including chipping of the veneering ceramics.

Radiographs were examined to record cement excess mesially and/or distally at the implants and

to evaluate marginal fit of the crowns using a modified marginal adaptation score 73

ranging from 1

to 4: score 1 was excellent fit, 2 was distinguish misfit, 3 was distinct misfit, and 4 was

unacceptable misfit (Figure 4). The marginal adaptation score of each ISSC restoration

corresponded to the highest score detected on radiographs from both follow-up examinations.

Figure 3. The marginal bone level was determined by

measuring the distance between a reference point at the

top of the implant (R) and the most coronal bone-implant

contact (B).

The interproximal marginal bone width was measured as

the distance between R and S (surface of the neighbouring

teeth or implants) parallel to the occlusal plane (green

stippled line).

29

Aesthetic variables, professional-reported (study II, III and IV)

The aesthetic outcome of the ISSCs was evaluated by using the Copenhagen Index Score 73, 81

. In

study II, all six aesthetic parameters, i.e. crown morphology, crown colour match,

harmony/symmetry score, mucosal discolouration score and papilla index score, mesially and

distally were included. In study III and IV, five out of the six aesthetic scores (without the

harmony/symmetry score) were used. All scores varied from 1 for excellent to 4 for poor aesthetic

result. The overall professional-reported aesthetic outcome was expressed by summary of all

included scores, i.e. the six scores in study II and the five scores in study III and IV.

The aesthetic parameters were assessed by using the photographs taken at the follow-up

examinations (study II, III & IV) combined with the clinical registrations (study III & IV).

Furthermore, cast models fabricated before crown cementation were used to measure the mesio-

distal distance in the edentulous space (DES) as the minimum coronal distance between the

proximal surfaces facing the implant site.

Figure 4. Radiological illustration of

marginal adaptation score 1 to 4

30

Validity and reliability of Copenhagen Index Score (study II)

The extra- and intra-oral photographs of the 66 implant-supported premolar crowns were used to

evaluate the six aesthetic parameters of the Copenhagen Index Score as well as VAS (Visual

Analogue Scale) scores—a 100 mm line with the end phrases ‗‗very bad aesthetic‘‘ on the left and

‗‗very good aesthetic‘‘ on the right. The VAS scores and summary of scores of six aesthetic

parameters (CIS score) were used to mark the overall impression of the aesthetic results.

One undergraduate dental student and two prosthodontists, one experienced and the other non-

experienced, evaluated all photographs twice with an interval of 1 week. In addition, 10 dental

students were randomly divided into two groups and asked to rate the aesthetic outcomes of the

crowns only once.

To test the convergent validity, the observer with the highest internal consistency marked the

general impression of each six aesthetic parameters used to define the current index separately on

the VAS.

Patient-reported outcomes (study II, III and IV)

A possible impact of oral health-related quality of life was evaluated by the patients using the

Danish version of the Oral Health Impact Profile questionnaire (OHIP-49) before the prosthetic

treatment and at the follow-up examinations (studies II & III). Each question answer was scored

with the Likert response scale from 0 (never experienced problem) to 4 (problem experienced very

often). The summary of questions 3, 4, 20, 22, 31 and 38 was used to describe the patient-reported

aesthetic outcome 73

(study II & III), and the masticatory function was expressed by the summary

scores of questions 1, 28, 29 and 32 95

(study III). The overall oral health impact on quality of life

was described by a summary of the scores from all 49 OHIP questions (study III). For patients with

more than one restoration, the mean of summary scores was used.

In study IV, the patients assessed the overall aesthetic outcome of each ISSC in the premolar

regions by a Visual Analogue Scale (VAS) at the 1-year examination.

31

Statistical analysis

The statistical analyses were performed with an SAS 9.1 package. The statistical significance level

was set at P < 0.05.

Study I

The initial and final fracture modes were analysed by descriptive analysis and Mann-Whitney test

by using ranks corresponding to increasing severity. The Cox proportional hazards analysis where

used to analyze the differences in loading cycles until fracture mode ≥ 3a.

Study II

Reliability

To test the reliability of Copenhagen Index Score, intra-observer agreements and weighted Cohen‘s

κ were calculated for the experienced and non-experienced prosthodontists, and for the

undergraduate dental student.

The inter-observer agreements were calculated between (i) experienced prosthodontist and non-

experienced prosthodontist (ii) experienced prosthodontist and student, (iii) non-experienced

prosthodontist and student and (iv) two groups of five students.

Additionally, stability was tested by calculating the mean of intra- and inter-observer Cohen‘s k

for pooled parameters, and the internal consistency was analysed by the Cronbach α.

Validity

The Spearman‘s test was used to correlate the overall aesthetic results measured by VAS to the CIS

values. To test the convergent validity, the six aesthetic parameters were correlated to the

corresponding VAS scores.

Study III and IV

To account for the correlation between several restorations applied to the same patient, models had

to incorporate a random subject level. For the quantitative data (e.g., marginal bone loss and DES),

evaluation was performed by using a traditional mixed model of ANOVA. For ordinal categorical

data (differences in mPlI, mBI, marginal adaptation score and professional-reported aesthetic

scores), a nonlinear mixed model was applied using PROC NLMIXED 96

.

The logistic regression model was used to analyze the relation between the presence and absence

of apical root resorption at neighbouring teeth and the number of tooth agenesis for patients who

received orthodontic pretreatment (study III).

32

Patient-reported outcome

To analyze the difference of aesthetic outcomes in patients with different restoration materials, the

non-parametric one-way ANOVA was performed (study III). In study IV, the difference in patient-

reported aesthetic VAS scores between AC and MC restorations (excluding the harmony/symmetry

score) was analyzed by using mixed model of ANOVA.

To analyse the correlation between the professional- and patient-reported aesthetic outcomes, the

Spearman‘s correlation analysis was performed (study II & III).

33

SUMMARY OF RESULTS

Study I

Fracture mode during cyclic loading of implant-supported single-tooth restorations

In this in vitro study, veneering fracture was the most frequently observed fracture mode for the AC

as well as the MC restorations. All MC restorations resisted 4.2 million cyclic loadings without

coping and/or abutment fracture. In contrast, 6 out of the 16 AC restorations did not resist 4.2

million cyclic loadings as they fractured in coping and abutment.

The statistical differences in fracture modes and number of loading cycles until veneering

fracture (≥3a) between restorations of various materials are demonstrated in Table 6. Significantly

more loading cycles until the veneering fracture were estimated, when the MC-I restorations were

compared to the AC-I and MC-H restorations.

Figure 5 illustrates that, although no significant difference in the number of loading cycles

between the AC and the MC restorations was detected, more loading cycles were needed before the

MC restorations fractured in the veneering ceramics.

Table 6. P-values for Mann-Whitney test for distribution of initial and final fracture modes (using

ranks corresponding to increasing severity), and for Cox proportional hazards analysis to estimate

the differences in loading cycles at fracture mode ≥ 3a

Initial fracture mode

(P-value)*

Final fracture mode

(P-value)*

Loading cycles until

fracture mode ≥ 3a

(P-values)#

AC-H vs. MC-H 0.720 0.061 0.592

AC-I vs. MC-I 0.003 0.007 0.038

AC vs. MC 0.010 <0.001 0.161

AC-H vs. AC-I 0.791 0.238 0.565

MC-H vs. MC-I 0.019 0.049 0.036

* Mann-Whitney analysis

# Cox proportional hazards analysis

AC: All-ceramic restoration (AC-H and AC-I)

MC: Metal-ceramic restoration (MC-H and MC-I)

AC-H: AC restorations veneered with HeraCeram Zirconia

AC-I: AC restorations veneered with IPS e.max Ceram

MC-H: MC restorations veneered with HeraCeram

MC-I: MC restorations veneered with IPS d.SIGN

34

Figure 5. Estimated number of cyclic loadings until fracture of the veneering

ceramics (fracture mode ≥ 3a); all-ceramic (AC, n=16) vs. metal-ceramic

(MC, n=16) restorations

35

Study II

A feasible, aesthetic quality evaluation of implant-supported single crowns: an analysis of

validity and reliability

Reliability

The intra-observer agreement and weighted Cohen´s are presented in Table 7. The mucosal

discolouration score had generally the highest observed agreement, and the crown morphology

rated by prosthodontists and the distally papilla index score evaluated by the student had the lowest

frequency of agreement.

The weighted Cohen´s demonstrated that the highest intra-observer agreement was for the

papilla index score, mesially, evaluated by both prosthodontists (substantial), and for the crown

colour match evaluated by the student (substantial). The intra-observer agreement was substantial

for the mucosal discolouration score for all observers.

The Cronbach α for the experience prosthodontist, non-experienced prosthodontist and

undergraduate student was 0.84, 0.87 and 0.85, respectively.

Table 8 demonstrates the inter-observer agreement and weighted Cohen´s . The mucosal

discolouration score had the highest frequency of inter-observed agreement and the highest Cohen´s

(moderate in all observations). The mean of intra- and inter-observer Cohen´s for pooled

parameters was 0.53 (stability test).

Validity

A significant correlation between the CIS and the overall VAS scores were observed (Table 9). The

six aesthetic parameters showed a highly significant correlation to the corresponding VAS scores.

Patient- and professional-reported aesthetic outcomes

No significant correlations between the overall professional VAS scores and CIS to the summary

scores of the six OHIP questions were found.

36

Table 7. Intra-observer agreement and Cohen´s (weighted) for all 6 aesthetic parameters

Assessment I vs. II

Experienced Non-experienced Student

Parameters

Observed

agreement

(%)

Cohen‘s

Observed

agreement

(%)

Cohen‘s

Observed

agreement

(%)

Cohen‘s

Crown morphology 72.2 0.63 68.2 0.52 66.7 0.54

Crown color match 80.3 0.32 74.2 0.61 84.1 0.72

Symmetry/harmony 79.3 -0.04 78.1 0.64 59.7 0.56

Mucosal discolouration 84.8 0.66 81.8 0.70 84.1 0.69

Mesial papilla 78.8 0.72 80.3 0.76 66.7 0.57

Distal papilla 72.7 0.53 68.2 0.59 50.0 0.46

All six parameters 75.6 0.64 75.1 0.67 68.6 0.63

Table 8. Inter-observer agreement and Cohen´s (weighted) for all six aesthetic parameters

Experienced vs. Non-

experienced Assessment I & II

Experienced vs.

student Assessment I & II

Non-experienced vs.

student

Assessment I & II

Two student groups

Assessment I

Parameters

Observed

agreement

(%)

Cohen‘s

Observed

agreement

(%)

Cohen‘s Observed

agreement

(%)

Cohen‘s Observed

agreement

(%)

Cohen‘s

Crown morphology 49.2 0.23 53.8 0.32 49.3 0.27 47.4 0.38

Crown color match 59.1 0.25 53.8 0.15 63.6 0.44 50.0 0.31

Symmetry/harmony 62.9 0.15 40.2 0.15 57.8 0.45 47.9 0.33

Mucosal discolouration 75.7 0.51 81.1 0.59 79.5 0.60 63.8 0.53

Mesial papilla 54.5 0.40 60.6 0.50 66.6 0.56 56.2 0.44

Distal papilla 58.3 0.40 45.8 0.39 52.7 0.42 61.7 0.47

All 6 parameters 60.0 0.42 56.5 0.43 60.6 0.51 54.0 0.42

37

Table 9. The six aesthetic parameters correlated to corresponding Visual Analogue Scale (VAS)

scores, and Copenhagen Index Score (CIS) correlated to the overall VAS score (n=66)

rs 95% CI P

Crown morphology -0.54 -0.7 to -0.35 <.0001

Crown colour match -0.63 -0.77 to -0.46 <.0001

Symmetry/harmony -0.79 -0.9 to -0.62 <.0001

Mucosal discolouration -0.57 -0.72 to -0.38 <.0001

Mesial papilla -0.77 -0.86 to -0.65 <.0001

Distal papilla -0.62 -0.76 to -0.45 <.0001

CIS -0.61 -0.75 to -0.43 <.0001

rs: Spearman‘s rank correlation coefficient; CI: Confidence interval

38

Study III

A comparative, 3-year prospective study of implant-supported, single-tooth restorations of

all-ceramic and metal-ceramic materials in patients with tooth agenesis

Biological variables

All implants survived after 3 years and only one implant with a marginal bone loss of 2.5 mm did

not fulfil the radiographic success criteria. The measured marginal bone loss was generally low but

significantly (P=0.040) higher at implants supporting the gold alloy abutments (0.41 mm, SD 0.58)

compared to those supporting the zirconia abutments (0.15 mm, SD 0.25) (Figure 5). At the 3-year

examination, 2 buccal marginal and 3 buccal apical fistulas were registered (Figure 6).

Figure 6. Buccal marginal fistula at

baseline (a), reduced at the 3-year

observation (b).

Buccal apical fistula in combination with

exfoliation of bone graft materials at the

baseline (c), reduced at the 3-year

observation (d)

Figure 5. Mean, 2SD and outliers

of marginal bone loss at sites with

gold alloy, titanium and zirconia

abutments

39

Fifty out of 59 patients had received orthodontic pretreatment, and the apical root resorptions

(score 2 or more) were registered in at least one neighbouring tooth to the implants in 31 of these 50

patients (62%). None of the 9 patients without a history of orthodontic pretreatment demonstrated

apical root resorption.

Biomechanical and technical variables

The survival rate of the abutments and crowns was 97%. There was registered: 3 crowns with loss

of retention (3 MC restorations; all recemented), 2 fractures of veneering ceramic (2 AC

restorations; 1 polished and 1 remade) and 2 unacceptable marginal adaptations (1 AC and 1 MC

restorations; both remade).

Excesses of cement materials were observed at 4 ISSCs, where the marginal bone loss was only

demonstrated at one of these restorations with a marginal adaptation score 2. No significant relation

between cement excess and mBI was found. Marginal adaptation scores were significantly lower at

the metal-ceramic compared to the all-ceramic crowns (P= 0.020).

Professional-reported aesthetic variables

No significant differences in the crown morphology scores, the mucosal discolouration scores and

the papilla index scores, mesially and distally between the all-ceramic and the metal-ceramic

restorations were observed. The scores of the crown colour match were significantly (P= 0.015)

lower at the all-ceramic than at the metal-ceramic crowns.

While the frequency of score 1 for the mucosal discolouration decreased, it increased for the

papilla index scores at the zirconia and the metal abutments from the baseline to the 3-year

observation.

Patient-reported variables

The patient-reported satisfaction with aesthetic outcome, masticatory function and overall oral

health impact on quality of life increased, i.e. the OHIP scores decreased, during the course of the

study.

The means of summary scores on six aesthetic OHIP questions were not significantly different

between patients treated with various restoration materials.

The professional- and patient-reported aesthetic outcomes at the 3-year follow-up were not

significantly correlated (rs=0.21, P=0.18).

40

Study IV

A 1-year randomised, controlled trial of implant-supported, single-tooth restorations based on

zirconia versus metal-ceramic

Biological variable

All implants survived and no mobility was recorded after one year of function. The mean marginal

bone level at the baseline examination was significantly (P= 0.034) more apically positioned at the

AC (mean 0.58 mm, SD 0.62) than at the MC restorations (mean 0.33 mm, SD 0.33). At the 1-year

examination, the mPlI and mBI as well as the mean marginal bone loss was not significantly

different at the AC and the MC restorations (AC: mean 0.08 mm, SD 0.25, MC: mean 0.10 mm, SD

0.17).

At the 1-year examination, biological complications, e.g. fistula, exudation/suppuration, pain or

PPD≥ 5 mm, were detected at 10 restorations, 7 AC restoration, of which 5 had a marginal

adaptation score 2, and at 3 MC restorations, all with a marginal adaptation score 1 (Figure 7).

Biomechanical and technical variables

The survival rate of the abutments as well as the crowns was 98.7%. At the 1-year examination, 2

complications at 2 MC restorations were registered; one chipping of veneering ceramic (Figure 8)

and one loss of retention. The restoration with loss of retention was remade.

Cement excess was observed at 1 MC restoration with a marginal adaptation score 2 and a

marginal bone loss of 0.53 mm.

The marginal adaptation scores were significantly (P= 0.014) lower at the MC compared to the

AC restorations (Figure 9).

Figure 7. Marginal adaptation score 2 at all-ceramic (AC) restoration in region 34, and clinically

observation of suppuration at 1-year examination.

41

Professional-reported aesthetic variables

The crown morphology scores, mucosal discolouration scores, the papilla index scores, mesially

and distally, and CIS were not significantly different at the AC and the MC restorations.

The crown morphology scores increased significantly (P<0.0001) with higher DES values. The

frequency of the mucosal discolouration scores was almost unchanged for AC and MC restorations,

but the frequency of the papillae with score 1, mesially and distally increased at the AC and the MC

restorations from baseline to 1-year registration.

The crown colour match scores were significantly (P=0.031) lower at all-ceramic crowns than

at the metal-ceramic crowns.

Patient-reported aesthetic variables

The VAS scores were not significantly different between the AC and the MC restorations (AC:

mean 84.9, SD 18.4, MC: mean 83.1, SD 18.8).

No significant correlation between the CIS (professional-reported) and VAS (patient-reported)

was found. However, the VAS scores increased significantly with lower scores of the crown colour

match, crown morphology and papilla index, mesially.

Figure 8. Photographs of a metal-

ceramic crown (region 25) at the

baseline examination, and chip-

off fracture of the veneering

ceramic recorded at the 1-year

examination.

Figure 9. The frequency of the

marginal adaptation scores at

restorations with the all-ceramic

(AC) and the metal-ceramic

(MC) crowns.

42

DISCUSSION

Titanium abutments and metal-ceramic crowns have been used as ―golden standards‖ for

restorations supported by dental implants. Although other restoration material options have been

introduced and used in clinical settings, only few comparative in vitro and in vivo studies have

reported on different implant-supported restoration materials using traditional titanium abutments

and metal-ceramic crowns as control. The clinical studies in the present thesis intended to

investigate the clinical performance of zirconia-based implant-supported single-tooth restorations,

and the in vitro study was designed to estimate the biomechanical long-term results of these

restorations compared to titanium abutments and metal-ceramic crowns. Furthermore, the biological

outcome of gold alloy abutments was analysed. A feasible aesthetic index was tested for reliability

and validity, and it was used to assess the professional-reported aesthetic outcome of all-ceramic

crowns and abutments compared to metal-ceramic crowns and metal abutments. The patient-

reported aesthetic outcomes of implant-supported restorations were recorded and correlated to the

professional-reported aesthetic outcomes.

Influence of restoration materials on peri-implant tissue

Implant survival and success rate

In the present clinical studies with 1 and 3 year follow-up examinations, the survival rate of

implants in function was 100%, which is in accordance with the reported, estimated annual implant

failure rate varying from 0% to 2.5% 31

. After 3 years of oral function, the radiological success rate

of implants supporting titanium and zirconia abutments was 100%. Only one implant that supported

a gold alloy abutment and an all-ceramic crown with a radiological registered marginal misfit did

not fulfil the success criteria as the marginal bone loss was more than 1.9 mm after 3 years of

function 97

. The misfit of the all-ceramic crown could be one of the variables that influenced

marginal bone loss at this implant.

Marginal bone loss and plaque accumulation

The marginal bone loss during the present clinical studies was comparable at the zirconia and

titanium abutments after 1 and 3 years, but it was highest at gold alloy abutments. These findings

agrees with the experimental animal studies by Abrahamsson et al. 49

and Welander et al. 48

, which

also detected more marginal bone loss associated with gold alloy abutments, than at abutments of

43

oxide ceramics and titanium. In addition, in a systematic review of clinical studies of implant-

supported restorations by Sailer et al. 31

, the rate of marginal bone exceeding 2 mm was higher for

implants supporting metal than for those supporting ceramic abutments. In that systematic review,

the metal abutments referred to both gold alloy and titanium abutments, and ceramic abutments

referred to oxide ceramics, i.e. alumina and zirconia abutments.

In contrast, similar marginal bone loss at titanium and gold alloy abutments were reported in a

four-year clinical study of ISSCs 53

. It should be noticed that in our prospective clinical study, the

gold alloy abutments were mainly used in situations, where abutments were angulated to

compensate for buccal positioning of implants. This may have led to more marginal bone loss at the

implants supporting gold alloy abutments than at the implants supporting zirconia and titanium

abutments. It has also to be emphasized that the differences in marginal bone loss were small. The

overall annual marginal bone loss was less than 0.1 mm in both clinical studies, which agrees with

the corresponding values reported in a 10-year prospective study of Astra Tech implants by

Gotfredsen 3. Furthermore, the minor changes in the peri-implant marginal bone level during our

clinical studies were within the limits of variability for our radiological measurement method.

In order to compare the plaque accumulation on zirconia and titanium abutments, Rimondini et

al. 45

and Scarano et al. 52

reported on lower accumulation as well as colonization of bacterial

plaque on zirconia than on titanium surfaces. The amount of plaque in our clinical studies was

generally low and none of the abutments had a supramucosal exposure to the oral cavity. These

clinical conditions may be the reasons for no significant difference in plaque accumulation at

restorations with different abutment materials in the present clinical studies. This finding also

agrees with another clinical 3-year follow-up study of all-ceramic and metal-ceramic implant-

supported restorations 17

.

Biological complications

The most frequent biological complications at the marginal peri-implant soft tissue were fistulas

and suppuration observed during the 3-year prospective and the 1-year randomised clinical study,

respectively. Fistulas at implant sites have been associated with insufficient marginal adaptation of

crowns 76

, cement excess 4, apical pathology of neighbouring teeth

98 or screw loosening

99. The

apical buccal fistulas in the present prospective study were related to inflammatory reactions

originated from necrosis of the neighbouring tooth and to exfoliation of bone substitutes from

buccally augmented site. The buccal marginal fistulas as well as the suppuration in our clinical

44

studies were mainly registered at the restorations with all-ceramic crowns with suboptimal marginal

adaptation. It is well-known that marginal misfit at the crown and abutment interface may establish

a space for bacterial colonization and cause chronic inflammation and subsequent breakdown of the

surrounding tissue 100

.

Even though apical root resorption is a consequence of implant insertion, it is most likely a

complication induced by the orthodontic pretreatment. All patients in our clinical studies had tooth

agenesis, and the radiographic examinations in the prospective study demonstrated apical root

resorption in at least one neighbouring tooth to implants in 62% of patients who underwent

orthodontic pretreatments. The role of tooth agenesis as a factor to increase the risk of apical root

resorption during orthodontic treatment is not clear 101, 102

. However, the orthodontic treatment to

provide the required space for implants had a significant influence on the prevalence of apical root

resorption in our study. This finding is consistence with a comparable study of a similar group of

patients with tooth agenesis by Dueled et al. 73

.

Biomechanical and technical complication at different restoration materials

Crown and abutment survival and failure rates

The survival rates of crowns and abutments in the study III and IV were 96.9%, and 98.7%,

respectively. The annual failure rates of crowns and abutments in the study III and IV were 1.02%

and 1.33%, respectively, which are within the range of the estimated annual failure rate for ISSCs

reported by Jung et al. 25

.

When all ISSCs in both current clinical studies were pooled, the survival rate of the all-ceramic

restorations was 97.8% which was comparable with the survival rate of the metal-ceramic crowns

on metal abutments (98.6%). In the systematic review study by Sailer et al. 31

, the survival rate for

either types of restorations was 100%, however, the number and follow-up period of studies

reporting on the all-ceramic restorations were less than the studies reporting on the metal-ceramic

restorations. As all zirconia-based ISSCs in the posterior regions survived in our clinical studies as

well as in the study by Zembic et al. 17

, these restorations may be a suitable alternative to the

traditional metal-ceramic restorations also in the posterior regions at least for short-time.

No complications during the present clinical studies involved the abutments, but when crowns

were remade new abutments were used for practical reasons. In a study by Aboushelib & Salameh

103, a few clinical cases of zirconia abutment fractures were analysed, and it was assumed that the

45

over-reduction of axial wall thickness of abutments, incorrect position of abutments and tightening

of screws beyond the recommended torque as well as fabrication defects could contribute to

zirconia abutment fractures. Generally, fracture of metal as well as ceramic abutments have been

reported as a seldom complication in clinical short-time studies 25, 31

. According to these systematic

reviews, screw loosening has been reported as the most common biomechanical and technical

complication of ISSCs. However, in our studies and in the clinical study by Zembic et al. 17

, no

screw loosening were observed which may be explained by the use of a torque wrench for

tightening of all abutment screws. In earlier prospective studies performed before the introduction

of torque wrenches higher frequencies of screw loosening have been reported 99, 104, 105

. The time

period may also have a great impact on the number of screw loosening.

Loss of retention, cement excess

The most frequent biomechanical complication in our clinical studies was the loss of retention. The

annual failure rate of this complication was 1.02% and 1.33% in study III and IV, respectively. The

loss of retention has also been reported as the second most frequent biomechanical complication of

ISSCs with an estimated annual rate of 1.13% 25

, and with no significant difference between crowns

supported by ceramic versus metal abutments 31

. Nevertheless, in our clinical studies, this

complication was mainly observed at metal-ceramic crowns in the posterior regions. The lower

height of the abutments in these regions and consequently minor mechanical retention as well as

higher forces and moments acting in the posterior regions 95

may have contributed to the occurrence

of this complication.

The radiological registrations in our clinical studies demonstrated five ISSCs with sub-mucosal

excess of cement materials. Marginal bone losses at sites with excess of cement were only observed

at restorations with suboptimal marginal adaptation of crowns and no significant relation between

cement excess and mucosal inflammation measured with mBI was found. In other clinical studies,

the cement excess has been related to the occurrence of fistulas 4 and to the clinical signs of the

peri-implant diseases 106

. However, in our studies, the presence of cement excess was only

registered radiographically, and the amount of cement excess registered at the radiographs was

limited.

46

Marginal adaptation

The radiological assessments of the marginal adaptation in our clinical studies demonstrated that the

frequency of misfit at the interface between the crowns and abutments was significantly higher at

the restorations with zirconia-based, all-ceramic crowns than at those with metal-ceramic crowns. In

an in vitro study by Tao & Han 107

as well as in a clinical study by Reich et al. 108

, the marginal gaps

at zirconia-based, all-ceramic restorations were greater than at metal-ceramic restorations, which

agrees with the results of the present clinical studies. However, contrasting results were reported in

another laboratory study by Gonzalo et al. 109

.

One of the explanations of the differences in the marginal adaptation between the all-ceramic

and metal-ceramic crowns in the current studies may be linked to the differences in the fabrication

procedures of these crowns. The enlarged pre-sintered zirconia copings should be sintered after the

milling process to obtain the final strength, which results in shrinkage of the material. As this

process is sensitive, it may result in deformation of restoration and marginal adaptation 110

.

Additionally, the subsequent porcelain veneering process may also have influenced the marginal

adaptation of the zirconia copings 111-113

.

Marginal adaptation may, however, also be related to oblique seating of crowns 114

and the clinical

procedures with modelling of the tight proximal contacts 115

. This should however be the same

procedure for the all-ceramic and the metal ceramic crowns.

Veneering fracture

In the present clinical studies, the annual rate of veneering fracture was 0.68% after 3 years (study

III) and 1.33% after 1 year (study IV) of function, which were almost in the same range as the

estimated annual rate for this complication reported by Jung et al. 25

. In our clinical studies, the

veneering fractures were registered at one metal-ceramic and at two all-ceramic restorations. In

agreement with our results, a higher rate of veneering fracture was reported in studies of all-ceramic

crowns than studies of metal-ceramic crowns 25

.

Laboratory test method

In spite of a high frequency of veneering fractures reported in short-term clinical studies of ISSCs

25, 31, 41, 42, the long-term clinical performance of zirconia-based abutments and crowns are still

unknown. In the present in vitro study, a clinical relevant laboratory method, which was developed

to induce veneering fracture, was used to compare the fracture of the all-ceramic and metal-ceramic

47

restorations during simulated long-term masticatory function. The method was set up to avoid

abutment, abutment screw and implant fractures, which are frequently observed in the laboratory

studies using load-to-fracture tests 37-40

. All zirconia abutment fractures in the current study

occurred as a consequence of coping fractures and were preceded by veneering fractures. The use of

loading force of 800 N was high, but within the range of maximal measured bite forces 95, 116, 117

.

The 4.2 million cyclic loadings corresponds to at least 16 years of clinical function 118

, or according

to Kelly 34

, this number of loading cycles at 800 N represents at least four years of constant bruxism

under extreme load. In a number of laboratory studies using lower loading forces or less number of

loading cycles, no fractures were developed 37, 40, 119

. It may be argued that the frequency of extreme

high loadings used in the present study were more than most subjects will experience. Additionally,

abutment fractures were very seldom registered with the used loading direction, which can be

interoperated as a good long-term strength of the titanium as well as the zirconia abutments. In an in

vitro study by Cho et al. 120

, it was demonstrated that the fracture strength of ISSCs was

significantly higher at the metal-ceramic than at the all-ceramic restorations, and fracture strength

increased significantly at a vertical compared to an oblique loading direction. Furthermore, the

fractographic analysis in an in vitro study by Aboushelib et al 121

demonstrated that the chip off

fractures were caused by e.g. surface defects, improper support by substructure or overloading and

fatigue. Although the restorations were exposed to high loading forces and fatigue in our in vitro

study, the low frequency of chip off fractures could be linked to the loading direction and the

structural support of veneering ceramic.

Fracture mode

In the present in vitro study, the fracture modes and the number of cyclic loadings until the

veneering fractures were influenced by the different core and veneering materials. The most

frequent fracture modes, i.e. the veneering fractures, were more severe at the all-ceramic than at the

metal-ceramic restorations. Furthermore, more loading cycles until the veneering fractures were

registered at the metal-ceramic than at the all-ceramic restorations. These results could be explained

by a lower bonding strength at the zirconia-ceramic than at the metal-ceramic interface and

indicated that the core-veneer interface in zirconia-based restorations was the weakest part of these

restorations 122, 123

. However, the difference in bonding strength between core materials of zirconia

and metal to the veneering ceramics was not significant in another in vitro study 124

. In addition to

48

insufficient bonding between veneering and coping, the fractures of veneering ceramics have been

assumed to be related to residual stress after firing and to polishing of veneering ceramics 44, 125, 126

.

Differences in bonding strength between different veneering ceramics to the same core materials

have also been reported. Thus, in a laboratory study, the bonding strength of the glass-ceramics was

higher than the bonding strength of the feldspathic ceramics to the metal core materials 127

. In the

current study, the fractures of restorations with feldspathic veneering ceramics were more severe

than those veneered with glass-ceramics; however, this difference was not demonstrated in a recent

laboratory study using another test method 27

. As a result of a limited number of restorations used in

the present in vitro study, the interpretation of the results should be done with caution.

Aesthetic parameters at different crown and abutment materials

The professional-reported aesthetic outcomes of all-ceramic and metal-ceramic restoration in the

clinical studies in this thesis included five aesthetic parameters. The reliability and validity of these

parameters in addition to the score of symmetry/harmony were analysed.

Reliability of aesthetic parameters

Reliability is a basic requirement of scales and it plays an essential role in judgment of adequacy of

any measurement process 75

. The acceptable level of reliability is not clearly defined and should be

related to the clinical situation and the measured variable 75

. The six aesthetic parameters in our

studies were based on the standards for an aesthetic fixed implant restoration 55

, i.e. the harmony

with the perioral facial structures was defined by using the symmetry/harmony score, the aesthetic

of peri-implant tissue was defined by the mucosal discolouration score and the papilla index scores,

and the natural appearance of the restorations by the scores of crown colour match and crown

morphology. The crown morphology and colour match scores used in our studies were adapted

from the CDA criteria 67

. The crown morphology and symmetry/harmony scores included several

sub-parameters and demonstrated a relatively low reliability in study II. Although a better test–

retest reliability could have been achieved with more unambiguous definition of these scores, e.g.

by separation into more parameters, this will result in a less feasible index. In the ICA index 66

, the

crown morphology was divided into several parameters, however, this scale also demonstrated

limitations in reliability 79

.

49

The crown colour match score used in the present studies had a relatively high intra-observer

agreement. In the WES, developed by Belser et al. 72

, this parameter composed of the score for

colour (hue/value) and the score for translucency/characterization, but the reliability and validity of

these scores were not analysed. In our study, the high reliability of crown colour match score may

be caused by the quality evaluation of crowns performed by the treating dentists. As the crowns

with suboptimal or poor colour were sent to colour corrections, it could led to a high frequency of

crowns with colour match score 1 and 2. For crown morphology score, however, the mesio-distal

space for each crown was an important factor and might have restricted the quality evaluation made

by the dentists.

The mucosal discolouration parameter had generally the highest reliability compared to the other

parameters. In the study by Jemt 70

, the discolouration of the soft tissue above the restoration as well

as visible titanium margins were identified as present or not present. Such a dichotomous scale is

feasible, but it may lead to a loss of efficiency 75

.

The evaluation of the mesial and distal papilla in the present study was based on the papilla

index score introduced by Jemt 70

as a simple clinical technique to assess recession or regeneration

of the interproximal soft tissue. However, the scores were reduced to four and turned around to

match the other aesthetic scores in the Copenhagen Index Score. In accordance with the present

study, Jemt 70

found a relatively good reproducibility of the papilla index score. The measurements

of the distal papilla score in study II were slightly less reliable than the measurements of the mesial

papilla score. This result could be related to practical limitations to reproduce the distal papilla by

photographs in the premolar region, which was in accordance with the results reported by Fürhauser

et al. (2005).

Validity of aesthetic parameters

The validity of the six aesthetic parameters was assessed in study II to evaluate the usefulness of

these parameters to measure the aesthetic outcome of single-tooth implant restorations. The

convergent validity of the parameters was analysed by using the VAS, which is the most frequently

used interval scale for dental, dentofacial or facial aesthetics 78

. The simplicity in using a categorical

scale and the definition of each score were the major advantages of using a categorical scale

compared with an interval scale such as VAS 75

. Furthermore, calibration of different observers is

more efficient using a categorical scale compared to an interval scale, which makes the categorical

scale more feasible for clinical evaluations. Some of the recently developed categorical scales for

50

assessing the aesthetic outcome of implant-supported restorations 59, 66, 70, 79, 80, 82

have also been

tested for reproducibility, but the validity was not evaluated.

Professional-reported aesthetic outcome

Using the five aesthetic parameters in the current clinical studies demonstrated that only the colour

match score varied at different restoration materials. A significantly superior colour match of the

restorations with the all-ceramic crowns compared to those with the metal-ceramic crowns was

observed. In another comparative study of ISSCs by Gallucci et al. 18

, the colour and translucency

of the all-ceramic and metal-ceramic crowns did not vary significantly, and the translucency of all

crowns was lower than the natural neighbouring teeth; however, the number of included crowns in

that study was very limited.

The morphology of the all-ceramic and the metal-ceramic crowns in our clinical studies was

comparable, which is in agreement with findings by Gallucci et al. 18

. The width of the crowns,

which was one of the subparameters of crown morphology score, had a significant influence on the

assessment of this aesthetic parameter in the study IV. In the edentulous regions, where the mesio-

distal distances were greater than the corresponded anatomic crown width of natural premolars 128

,

the morphology of crowns was generally less optimal. This was mainly observed, where the two

adjacent missing teeth were replaced with only one implant, and where an ISSC replaced a retained

primary second molar.

The zirconia abutments were primarily used to reduce the greyish discolouration of the marginal

peri-implant mucosa. However, in our clinical studies as well as in the other clinical studies 17, 61

,

the zirconia and the metal abutments induced no significant differences in the colour of the

marginal peri-implant mucosa. In an in vitro study by Jung et al. 14

, it was suggested that in clinical

situations with a mucosa thickness ≤ 2 mm, the titanium abutments in contrast to zirconia abutments

may cause a change in colour of the peri-implant mucosa. In a recent clinical study by Bressan et al.

129, the change in the peri-implant mucosa colour was significantly less at the zirconia than at the

titanium abutments, but the results were not depended on the mucosa thickness.

The height of the papilla in the present clinical studies as well as in the other clinical studies 17,

130 was similar at the titanium and the zirconia abutments. The papilla height increased during the

current studies, which is consistent with the results of the other clinical studies 131, 132

. The gingival

biotype 132

and the cervical dimension of the permanent healing abutments 133

are some of the

factors that have been suggested to influence the dimension of the interproximal papilla.

51

Patient-reported outcome

In the present prospective study patients with tooth agenesis treated with ISSCs reported on very

few oral health related problems three years after crown insertions, which is consistent with the

results of the other studies 3, 134

. In the present clinical studies, the patients did generally not notice

considerably differences in the aesthetic outcome of the all-ceramic compared to the metal-ceramic

restorations. This finding agrees with the results reported in a study by Gallucci et al. 18

.

In the current clinical studies as well as in other studies of ISSCs 3, 65, 73, 76, 87, 135

, the patients and

the clinicians had significantly different aesthetic views to the performed restorations, which

emphasizes the importance of involving patients in treatment planning and evaluation.

CONCLUSION

Fracture of the veneering ceramic appears to be the most frequent fracture mode of

ISSCs. More cyclic loadings until veneering fracture was estimated with the metal-

ceramic than the zirconia-based all-ceramic restorations (study I).

The six aesthetic parameters included in the CIS had an overall substantial intra-observer

and moderate inter-observer agreement. No significant correlation between the

professional- and patient-reported aesthetic outcomes was observed (study II).

Generally, minor marginal bone loss was observed at implants supported zirconia,

titanium and gold alloy abutments. No differences in marginal bone loss were registered

between sites with zirconia and titanium abutments after short-time follow-ups. The sites

with angulated gold alloy abutments had more marginal bone loss than the sites with

zirconia and titanium abutments. The health of the peri-implant soft tissue was not

influenced by the abutment materials (study III & IV).

More optimal marginal adaptation was achieved at metal-ceramic than at all-ceramic

crowns. Loss of retention was the most frequent biomechanical complication, which was

registered at metal-ceramic restorations mainly in the posterior regions. Only few

fractures of veneering ceramic were registered (study III & IV).

The professional-reported aesthetic outcome demonstrated that implant-supported all-

ceramic crowns provided a better colour match than metal-ceramic crowns. The crown

morphology was influenced by the mesio-distal distance in edentulous space (study IV),

and the mucosal discolouration (study III) as well as the papilla level (study III & IV)

52

increased. However, the restoration materials had no impact on the crown morphology,

mucosal discolouration and papilla index scores after short-time observations. The

patients did generally not noticed aesthetic differences between all-ceramic and metal-

ceramic restorations (study III & IV)

53

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